The present invention resides in an apparatus and method for an automated cetane number determination by means of a constant-volume combustion chamber including a fuel injection nozzle, means for filling the combustions chamber with compressed air, means for heating the combustion chamber and means for recording the pressure in the combustion chamber during ignition and combustion of sample fuel injected into the combustion chamber.
The cetane number is a measure for the ignition quality of intermediate distillates, particularly diesel fuel.
Examples for intermediate distillates are refinery products with a boiling range from about 150 to 500° C., which are commercially available as heating oil, diesel fuel, kerosene or jet fuel. Examples for bio-diesel fuels are fatty and methyl ester manufactured from rape oil, soybean oil, palm oil, suet, sun flower oil, waste food fats and mixtures thereof.
A good ignition quality of a diesel fuel means good start-up behavior, quiet operation of the diesel engine and good exhaust gas emission values.
The cetane number is defined by the norm EN ISO 5165 and its determination is described by that norm. Cetane numbers are usually determined in a standardized four cycle one-cylinder engine with variable compression ratio and indirect fuel injection. Since this engine-based method however is time-consuming and expensive attempts have been made to determine the cetane number by means of a constant volume combustion chamber method.
In a constant volume combustion chamber apparatus for determining the cetane number the measuring cycle comprises the following operating steps:
1. Filling a combustion chamber with compressed air and heating the combustion chamber,
2. injecting fuel into the combustion chamber via a fuel injection nozzle,
3. recording the combustion chamber pressure curve resulting from the fuel ignition and combustion, and
4. calculating the cetane number from the ignition delay measured.
The time delay between the fuel injection and ignition, that is, the ignition delay is measured as accurately as possible. This ignition delay is for conventional diesel fuel in a constant volume apparatus typically 3 to 10 ms. By measuring fuels with known cetane numbers under identical conditions, the apparatus can be calibrated.
The known constant volume apparatus have various disadvantages:                The use of mechanical or mechanical-electronic sensor elements which may have an actuation tolerance of up to 2 ms results in an insufficient precision in the ignition delay determination. It is tried to eliminate this disadvantage by the use of a multitude of measuring cycles with subsequent statistical evaluation.        The use of conventional injection nozzles which operate with comparatively low pressures makes the determination procedure sensitive with respect to changes in the surface tension of otherwise identical fuel samples.        With the use of conventional pump-nozzle systems, the fuel injection volume is not accurately known and can vary substantially. As a result, also statistical variations in the ignition delay occur.        The use of injection nozzles which are not temperature-controlled results in a drift of the measuring results until the injection nozzle has reached a thermal equilibrium.        For the testing of several fuel samples an automated testing procedure is not available.        In the fuel flow structures from the suction side to the injection nozzle, the known apparatus include large dead volumes. Consequently, large sample volumes are required for the cetane number determination.        
It is the object of the present invention to provide an improved and more accurate and automated cetane number determination for a constant volume apparatus.